Pressure pulsation frequency indicator



Jan. 31, 1961 c. J. COBERLY 2,969,676

PRESSURE PULSATION FREQUENCY INDICATOR Original Filed June 3, 1950 Fig. 1.

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CLARENCE d. CO5RLY BY HIS HTTORNEYS- IECH, F05 TEA 9c HARRIS RIP/5: a w m.

United States Patent PRESSURE PULSATION FREQUENCY INDICATOR Clarence J. Coberly, San Marino, Calif., assignor, by mesne assignments, to Kobe, Inc., Huntington Park, Calif., a corporation of California Original application June 3, 1950, Ser. No. 166,00,

now Patent No. 2,812,662, dated Nov. 12, 1957. D1- vided and this application Apr. 29, 1957, Ser. No.

8 Claims. (Cl. 73388) ing fluid, such as oil, is delivered to the fluid-operated pump under pressure by way of a supply line which extends from a source of pressurized operating fluid at the surface of the ground downwardly in the well to the pump. Such a fluid-operated pump includes a motor piston which is reciprocated in its cylinder by an alternating fluid pressure differential applied thereto, and includes a valve for so regulating the delivery of the operating fluid to the motor cylinder and the exhaust of spent operating fluid therefrom as to reverse the fluid pressure diiferential each time the motor piston reaches the ends of its stroke. In such a fluid-operated pump, the delivery of the operating fluid to the motor cylinder from the supply line is interrupted momentarily each time the motor piston reaches the ends of its stroke, with the result that the fluid pressure in the supply line increases momentarily, assuming that the rate of delivery of the operating fluid under pressure to the supply line is substantially constant, which is normally the case.

The magnitude and frequency of such pressure pulsations in the supply line of a fluid-operated pumping system are significant for various reasons. For example, the magnitude of the pressure pulsations provides an indication of the condition of the pump and also the pumping conditions in the well, while the frequency of such pulsations provides an indication of the stroke frequency of the pump. Since the pressure of the operating fluid employed in such a fluid-operated pumping system may be of the order of, for example, two thousand pounds per square inch, it will be apparent that any device for indicating or measuring the absolute pressure in the supply line will not provide readily perceptible indications of such pressure pulsations, or their frequency, since the pulsations may be of the order of from only twenty-five to fifty pounds per square inch, for example, relative to the normal pressure of the operating fluid.

In view of the foregoing, a primary object of the invention is to provide a pulsation frequency indicator which is capable of producing amplified and therefore readily perceptible responses to such pressure pulsations relative to an average or reference pressure, the average pressure depending upon the pump design and the fluid level of the well on which the indicator is employed. I

Another object is to provide a pulsation frequency indicator which may be used over a wide range of average or reference pressures and which will have a response sations.

or sensitivity substantially independent of the reference pressure. I

Liquids, such as the oils customarily employed in fluid-operated pumping systems, are normally regarded as substantially incompressible. However, such liquids do vary in volume with variations in pressure, the compressibility of the oils normally employed in fluid-operated pumping systems being in the neighborhood of one percent at the operating pressures normally employed, e.g., two thousand pounds per square inch.

With the foregoing in mind, a basic object of the invention is to provide a pulsation frequency indicator which utilizes the compressibility of a liquid to provide indications or measurements of the frequency of variations in pressure from a reference pressure.

An important object of the invention is to provide a pulsation frequency indicator responsive to the frequency of pressure pulsations applied to a piston, the piston being disposed in a cylinder one end of which communicates with a chamber containing a compressible liquid so that the pressure pulsations result in inverse variations in the volume of the liquid at the frequency of the pressure pulsations.

Another object is to provide a pulsation frequency indicator having electrical means for transforming the frequency of movement of the piston into an indication of the frequency of the pressure pulsations.

Another object is to provide a pulsation frequency indicator which operates completely hydraulically to transform the pressure pulsations applied to the piston into an indication of the frequency of the pressure pul- A related object is to provide a pulsation frequency indicator wherein each pressure pulsation moves the piston toward one end of its cylinder and wherein the piston moves toward the opposite end of its cylinder between pulsations so that the position of the piston in its cylinder represents the frequency of the pressure pulsations applied thereto. 7

The foregoing objects and advantages of the present invention, together with other objects and advantages thereof which will become apparent, may be attained with the exemplary embodiments of the invention which are illustrated in the accompanying drawings and which are described in detail hereinafter. Referring to the drawings:

Fig. l is a sectional view of one embodiment of a pulsation frequency indicator of the invention;

Fig. 2 is a fragmentary sectional view taken along the broken line 2-2 of Fig. 1;

Fig. 3 is a diagrammatic view of an electric circuit forming part of the pulsation frequency indicator of Fig. 1;

Fig. 4 is a diagrammatic view of another embodiment of a pulsation frequency indicator of the invention;

Fig. 5 is a plan view of a modification of a portion of the embodiment of Fig. 4; and

Fig. 6 is a sectional view taken as indicated by the arrowed line 6-6 of Fig. 5.

Referring first to Figs. 1 to 3, illustrated therein is an electric pulsation frequency indicator which relies on the compressibility of a liquid in its operation. This pulsation frequency indicator includes a cup-shaped housing -v the open end of which is closed by a plug 76 threaded, thereinto, the housing 75 and the plug 76 cooperating to define a chamber 77 which is filled with a compressibleliquid 78. Extending through the plug 76 is a bore which: provides a cylinder 79 for a piston 80, the inner-endof; this piston being exposed to the pressure of the liquid78a The outer end of the piston 80 is exposed to fluid pressure; in a chamber 81 which is defined by a counterbor' '2'; communicating with the cylinder 79, and by a diaphragm 83, the periphery of the latter being clamped between the" Patented Jan. 31, 1961 cates with a longitudinal passage 88 in an element89 threaded 'intoth'e fitting 84. The longitudinal passage 88 communicates with a transverse passage 90 in the element 89, the ends of the'tr ansverse passage 90 communicat ng with an annular space which is provided by a packing gland 92. The latter is retained by ahead 93 on thethreaded element 89 and provides a lateral passage 94 which communicates 'with'the annular space '91 and which is adapted to be connected, as by welding, for example, to'

a line 95 leading to the source of fluid pressure the frequency of pulsations in which is to be measured. For example, the line 95 maybe connected to the operating fluid supply line leading to a fluid-operated pump.

As will be apparent, any pulsations in the fluid pressure in the chamber 85 externalto the diaphragm 83 are communicated to the liquid in the chamber 81 interiorly of the diaphragm and are thus applied to the outer end of the piston 80. Consequently, the piston varies the volume of the liquid 78 in the chamber 77 inversely with such pressure pulsations and at the same frequency.

It will be noted that in the pulsation frequency indicator under consideration, the ratio of the volume of the chamber 77 to that of the cylinder 79 is large so that, as 'a result, the pressure pulsations applied to the outer end of-the piston 80 result in only very short strokes of this piston. The piston 80 operates a sensitive microswitch 100 which is disposed in the chamber 77 and which is also filled with the liquid 78, the microswitch being pivoted in the chamber 77 on a support 101 by a pivot pin-102. A

compression spring 103, seated at one end against the plug 76 and at its other end against a washer 104 carried by the piston 80 tends to bias the piston toward the microswitch, the washer 104 being retained by a nut 105 threaded onto the piston. The purpose of biasing the piston 80 in this manner is to maintain contact between the piston and the microswitch, the spring force being insuflicient to operate the microswitch. The spring force may be adjusted by a screw 106 seated against the microswitch and threaded through its support 101.

Connected to one side of the microswitch 100 is a lead 107 which is soldered or otherwise connected to a special bolt 108 having a threaded portion for a nut 109, the other-side of the microswitch being grounded. Disposed between the head of the bolt 108 and the nut .109 is a frusto-conical packing element 110 which is pressed into a tapered bore 111 in a plug 76, the packing element being adapted to provide a liquid-tight seal between the bolt 108 and the plug 76. Also, it will be noted that the tapered bore 111 and the packing element 110 converge outwardly so that the pressure of the liquid 78 tends to seat the packing element in the tapered bore to insure a liquid-tight seal between the packing element and the plug 76. The bolt 108 is provided with an extension 112 of reduced diameter which is the pin of an electrical connector 113, the latter having a lead 114 leading therefrom.

Referring particularly to Fig. 2, the chamber 81 interiorly of the diaphragm 83 also communicates with the chamber 77 through a passage 118, a throttle valve 119 and a bore 120, the throttle valve 119 comprising a valve element 121 threaded into the bore 120. The clearance between the external threads on the valve element 121 and the internal threads in the bore 120 provides a throttling passage the restriction of which may be'varied by'varying the position of the valve element 121 in the bore 120. As will be apparent, the throttle valve 119 serves as a means for compensatingvfor drift. No means for adjusting the sensitivity of response of the piston 80 tofluid pressure pulsation,i.e., the sensitivity. of amplitude response, is required in this embodiment.

If desired, the microswitch 100 may be connected in steamers"- series with an electric signalling device of any desired type. For example, the microswitch may be connected in series with an electric light, not shown, to energize the light at a frequency corresponding to the frequency of the fluid pressure pulsations being measured. Also, the microswitch 106 may be connected in series with an electric counter to count the number of pressure pulsations occurring in any desired interval of time, such.

an arrangement being illustrated in Fig. 3,, in a circuit which also includes a meter for indicating the pulsation frequency.

Referring to Fig. 3, the circuit illustrated diagrammatically therein includes a terminal 125 to which the lead 114 may be connected. The circuit also includes a terminal 126 and a terminal 127 which are rendered positive and negative by connecting thereacross a battery, not shown, for example. A main switch 128 is included in the circuit adjacent the terminal 126 in the particular arrangement illustrated. Connected in series with the terminal 125, the terminal 126, and the terminal 127 is a relay 1'31, i.e., the relay 131 is connected in series with the microswitch'100 and with the power source represented by the positive and negative terminals 126 and 127. Thus, the relay is energized at a frequency equal to the frequency of the fluid pressure pulsations to whichv electric counter in series with the power source repre sented by the positive and negative terminals 126 and 127, thereby energizing the counter 135 at a frequency equal to the frequency of the fluid pressure pulsations being measured. Thus, the counter 135 may be employed to count the number of pressure pulsations occurring in any desired interval of time.

The circuit of Fig. 3 also provides a meter which may be calibrated in terms of frequency and which indicates the frequency of the pressure pulsations applied to the piston 80. In effect, that portion of the circuit of Fig. 3 relating to the frequency indicating meter 140 is essentially a vacuum tube voltmeter which measures the voltage applied to a condenser 141. In order to make the voltage applied to the condenser 141 a function of the pulsation frequency, the microswitch 100 energizes the relay 131 at". the pulsation frequency to periodically charge a condenser 142 to the line voltage, i.e., the voltage of the power source represented by the positive and negative terminals 126 and '127, which may be of the order of magnitude of six volts. As will be apparent, the condenser 142 is connected across the power source represented by the terminals 126 and 127 through the relay-operated switches 132 and 134 each time the relay 131 is energized by closure of the microswitch 100 in response to a pressure pulsation. Thus, the condenser 142 is charged to the line voltage at the frequency of the pressure pulsations being measured.

Each time the relay 131 is de-energized, the charge applied to the condenser 142 is transferred to the condenser 141 by the switches 132 and 134 so that the potential applied to the condenser 141 ultimately would" increase to the line voltage. However, the condenser 141 is shunted with a grid resistor 143 which is connected to the negative terminal 127, thereby tending to discharge the. condenser 141. denser 141', therefore, is always a function of the frequency of application of voltage to the condenser 142, and therefore, is always a function of the frequency of the pressure pulsations being measured.

The remainingelemen'ts of the electrical circuits of Fig. 3 are for the purpose of adapting it to the particular vacuum tube 144 employed and for setting and checking the sensitivity and zero positionof the meter 140. These elements of the circuit form no part of the in- The voltage of the con-' vention per se and it is thought that a detailed description thereof is unnecessary.

As will be apparent, by employing the circuit of Fig. 3, and by connecting the line 95 of the pulsation frequency indicator of Figs. 1 to 3 to the supply line for delivering operating fluid to a fluid-operated pump, the meter 140 indicates the stroke frequency of the motor piston incorporated in the pump and the counter 135 indicates the number of strokes occurring in any desired interval of time. It will be understood, of course, that this is but one of many possible applications of the pulsation frequency indicator of Figs. 1 to 3.

Referring now to Fig. 4, illustrated therein is a pulsation frequency indicator which is entirely hydraulic and which also relies for its operation on the compressibility of a liquid. This pulsation frequency indicator includes a chamber 150 which communicates with the upper end of a substantially vertical cylinder 151 through a passage 152, the chamber also communicating with the lower end of the cylinder through a passage 153. A check valve 154 in the passage 152 permits flow from the cylinder 151 into the chamber 150 but prevents flow in the opposite direction, and a check valve 155 in the passage 153 prevents flow from the cylinder into the chamber 150, but permits flow in the opposite direction.

Communicating with the lower end of the cylinder 151 and with the passage 153 is a passage 156, the latter terminating in a chamber 157 which is separated from a chamber 158 by a diaphragm 159. Communicating with the chamber 158 exteriorly of the diaphragm 159 is a passage 160 which leads to a source of fluid pressure the frequency of pulsations in which are to be measured. For example, the passage 160 may communicate with the supply line for delivering operating fluid to a fluidoperated pump. Providing fluid communication between the ends of the cylinder 151 is a passage 161 having a throttle valve 162 therein.

Except for the chamber 158 and the passage 160, the entire instrument is filled with a compressible liquid, such as clear oil, the diaphragm 159 serving to separate such liquid from the fluid subject to pressure pulsations. The volume of the cylinder 151 is very small relative to the total volume of the liquid in the system, a typical example of the relative volumes being included hereinafter.

Disposed in the cylinder 151 is a piston 165, the cylinder preferably being transparent so that the position of the piston in the cylinder may be observed visually when clear oil. or other clear liquid, is used in the instrument. The position of the piston 165 in the cylinder 151 may be determined with reference to a scale 166 parallel to the axis of the cylinder.

In operation, fluid pressure pulsations applied to the diaphragm 159 are transmitted thereby to the liquid in the instrument, the pressure pulsations transmitted to the liquid being applied to the lower end of the piston 165. Each pressure pulsation applied to the lower end of the piston 165 moves it upwardly in its cylinder 151 a distance determined by the relative volumes of liquid in the cylinder and the instrument as a whole, thereby compressing the liquid inthe chamber 150. It will be noted that the check valve 154 opens each time the piston 165 is displaced upwardly to permit displacement of liquid from the cylinder 151 above the piston into the chamber 150, displacement of liquid into the chamber 150 through the passage 153 during each pressure pulsation being prevented by the check valve 155.

As soon as each pressure pulsation ceases the check valve 155 opens to relieve the pressure built up in the chamber 150. However, the check valve 154 closes as soon as each pressure pulsation ceases so as to avoid relieving the built-up pressure in the chamber 150 through the passage 152. Consequently, the piston 165 is subjected only to the influence of gravity during the intervals between pressure pulsations. Clearance is provided hetween the piston 165 and the cylinder 151 so that the piston may descend toward the lower end of its cylinder 151 at a constant rate during the intervals between pressure pulsations, the distance that the piston descends during the interval between a pair of pressure pulsations and the distance the piston is displaced upwardly by each pressure pulsation being so related that the position of the piston relative to the scale 166 represents the number of pressure pulsations occurring in a suitable interval of time, i.e., represents the pressure pulsation frequency. For example, the scale 166 may be calibrated in terms of the number of pressure pulsations occurring in an interval of one minute so that the position of the piston 165 relative to the scale represents the number of pulsations per minute.

The distance that the piston 165 is displaced upwardly during each pressure pulsation depends upon the magnitude of the pulsation, whereas the distance the piston descends during a given interval of time is constant. Thus, it will be seen that if the pulsation frequency indicator under consideration is designed to indicate a particular frequency for a particular pulsation amplitude, the indicator will not provide a correct frequency indication for pressure pulsations of the same frequency, but of a different amplitude. However, the throttle valve 162 serves as an adjusting means for compensating for this effect. For example, if the pulsation frequency indicator is employed first for measuring the frequency of pressure pulsations of a particular amplitude and is then employed for measuring the frequency of pressure pulsations of the same frequency, but of a higher amplitude, the throttle valve 162 may be opened to decrease the restriction to flow to the passage 161 so as to lessen the upward displacement of the piston 165 during the pressure pulsations. Thus, by means of the throttle valve 162, the upward displacement of the piston 165 during each pres sure pulsation may be adjusted to compensate for variations in pulsation amplitude. The adjustment of the throttle valve 162 may be made readily by first determining the pulsation frequency with a stop watch, for example, and adjusting the throttle valve 162 until the piston 165 indicates the stop watch frequency on the scale 166.

As an example of one possible application of the pulsation frequency indicator of Fig. 4, assume that the passage is connected to the supply line for delivering operating fluid to a fluid-operated pump. As previously discussed, pressure pulsations occur in the supply line to the pump at the ends of the stroke of the motor piston in the pump. Such pressure pulsations displace the piston upwardly in its cylinder 151 until it assumes a position relative to the scale 166 which corresponds to the stroke frequency of the pump. In order to adjust the pulsation frequency indicator to correspond to the calibration of the scale 166, the frequency of the pressure pulsations may first be determined with a stop watch, for example, by counting the number of upward displacements of the piston 165 in a given interval of time, e.g., one minute. Then, by means of the throttle valve 162, the indicator may be adjusted until the piston 165 assumes a position relative to the scale 166 corresponding with the frequency reading determined by means of the stop watch. Thereafter, so long as conditions remain constant, the position of the piston 165 relative to the scale 166 continues to indicate the pump stroke frequency. If desired, the device may be recalibrated from time to time to insure that the reading provided by the scale 166 is accurate and, if it is not, any inaccuracies may be compensated for readily by means of the throttle valve 162.

Considering an example of the relative volumes of the cylinder 151 and the liquid in the instrument, assume that the length of the cylinder is five inches to cover a range on the scale 166 of from zero to one hundred strokes per minute. There are two pulsations per stroke so there would be 200 pulsations per minute. Also assumethat the'magnitude of each pressure pulsation is ten pounds-per square inch and that the liquid employed in the instrument is compressed one'percent at a reference pressure of two thousand pounds per square inch. Further assume that the clearance between the piston 165 and the cylinder 151 and the viscosity of the liquid employed are such that the piston descends from the upper end of the cylinder to the lower end thereof in one minute under the influence of gravity. Consequently, in order to have a pulsation'frequency of two hundred per minute maintain the piston 165 at the upper end of its cylinder, the piston would have to be displaced upwardly by'the pressure pulsations a distance equal to twice the length of the cylinder, or ten inches. Additionally, assume that the volume of liquid employed is two cubic inches! Under these conditions, the diameter of the cylinder 151 would have to be 0.050 inch. It will be understood, of course, that the foregoing example is merely intended as illustrative and that there is no intention of limiting the invention thereto.

' It will be understood that while the cylinder 151 is illustrated in Fig. 4 as substantially vertical so that displacements of the piston 165 by the pressure pulsations are opposed by gravitational attraction for the piston, the cylinder may be mounted horizontally, for example, and means 170 for biasing the piston toward one end of its cylinder by magnetic attraction may be substituted, as shown in Figs. and 6. In this case, it is necessary to make the piston 165 of a magnetizable material and to provide a magnetic attraction which is constant throughout the length of the cylinder 151 so that the rate of movement of the piston toward one end of the cylinder between pulsations would be constant for any position of the piston in the cylinder. The biasing means 170 provides such constant magnetic attraction through the use of a channel-shaped magnet 172 having tapered pole pieces 174 on opposite-sides of the cylinder 151.

It will be noted that the pulsation frequency indicators of Figs. 1 to 3 and of Fig. 4 respond only to pressure increases from reference values. However, the pulsation frequency indicator of Figs; 1 to 3 may be made respon sive to pressure decreases from a reference value readily by so arranging the piston 80 and the microswitch 100 that the microswitch closes in response to outward, rather than inward, movement of the piston. Also, the indicator of Figs; 1 to 3 may be made responsive to both increases and decreases from a reference value by emplo'ying for the microswitch 100 a switch having two closed positions with a neutral, open position therebetween; Thus, the meter 140 would indicate the pulsation frequency, whether the pulsations be increases or decreases from the reference value. The pulsation frequency indicator of Fig. 4 may be made responsive to decreases in pressure from a reference value by inverting the instrument and by reversing the actions of the check valves 154 and 155, without inverting the scale 166.

Although I have disclosed exemplary embodiments of my invention herein and have indicated possible applica-' tions thereof, it will be understood that the invention is susceptible of other embodiments and other applications without departing from thespirit thereof as defined by the following claims.

I claim:

'1. In a device for measuring the frequency of variations in fluid pressure, the combination of: a rigid, liquidfilled chamber; a rigid cylinder communicating at-one end with'said' chamber; a rigid piston movable in said cylinder andhaving two opposed end surfaces one of which is exposed to the pressure of said liquid in said chamber; means, including rigid, liquid iilled passage means communicating with the other end 'of said cylinder, for exposing the other end surface of said piston to fluid pressure variations the frequency of. which is to be measured"; wherebythe fluid pres'su're variations result in movement of said piston at the frequency of such varia tions to vary the-volume of said liquid in said cham-' ber inversely with such variations and at the frequency thereof; and means for indicatingthe-frequency of movement of said piston.

2. In a device for measuring the frequency of varia tions in fluid pressure, the combination of: a rigid, liquid-filled chamber; a rigid cylinder communicating at one end with said chamber; a rigid piston movable in saidcylinder and having two opposed end surfaces one of which is exposed to the pressure of said liquid in'said chamber; means including rigid, liquid-filled passage means communicating with the other end of said cylinder, for exposing the other end surface of said piston to fluid pressure variations the frequency of which is to be measured, whereby the fluid pressure variations result in movement of said piston at the frequency of such variations to vary the volume of said liquid in said chamber inversely with such variations and atthe frequency thereof; and means for indicating the frequency of movement of said piston, including a switch actuable by said piston.

3. In a device for measuring the frequency of variations in fluid pressure, the combination of: a rigid, liquid lilled chamber; a rigid cylinder communicating at one end with said chamber; .a rigid piston movable-in said cylinder and having. two opposed end surfaces one of which is exposed to the pressure of said liquid in said chamber; means, including rigid, liquid-filled passage means communicating with the other end of said cylinder, for exposing the other end surface of said piston to fluid pressure variations the frequency of which is to be measured, whereby the fluid pressure variations result in movement of said piston at the frequency of such variations to varyth'e volume of said liquid in said chamber inversely with such variation and at the frequency thereof; and means for indicating the frequency of movement of said piston, including a circuit having therein a switchwhich is operatively connected to said piston so as to be actuable by said piston at the frequency of movement thereof, and having therein an indicator which is operatively connected to said switch so as to be energizable at the frequency of said switch.

4. In a device for measuring the frequency of variations in fluid pressure, the combination of: a relatively large, rigid, liquid-filled chamber; a relatively small, rigid, liquid-filled cylinder; a first rigid, liquid-filled passage connecting oneend of said cylinder'in fluid communicationwithcsaid chambenla second rigid, liquidfilled passage connecting the other end of saidcylinder in fluid communication withsaid chamber; first check valve means in said first passage for-preventing fluid flow from said chamber into said cylinder by way of said first passage; second check valve means irrsaid second passage for preventing fluid flow from said cylinder into said chamber by way of said second passage; a rigid pistonin said cylinder having opposed end surfaces one ofwhich faces said-one end of said cylinder and the other of which faces said other end thereof; means, -including a third rigid, liquid-filled passage communicating with said iother'end of said cylinder, for exposing said other-end surface of said piston to fluid pressure variations the frequency of which is to be measured; means for biasing said piston toward said other end of said cylinder; and'means for indicating the position of said piston in said cylinder.

7 S. -A device as defined in claim 4 wherein said cylinder is disposed generally vertically with said one end thereof uppermost, whereby gravity .biases said piston toward-saidother-endof said cylinder. I

6. Aqdcvice as defined in claim.4 wherein said cylinder is transparent, and .wherein said indicating -means comprises a scale-parallel to the axis of said cylinder.

7.. A device as defined in claim 4 including .a fourth rigid, liquid-filled-passag'e by-passing :said cylinder and by-pass -.valve meansin said fourthpassage.

'8. In a device for measuring the frequency of variations in fluid pressure, the combination of: a rigid, 1iquid-filled chamber; a rigid cylinder communicating at one end with said chamber; a rigid piston movable in said cylinder and having two opposed end surfaces one of which is exposed to the pressure of said liquid in said chamber; means, including rigid, liquid-filled passage means communicating with the other end of said cylinder, for exposing the other end surface of said piston to fluid pressure variations the frequency of which is to be measured, whereby the fluid pressure variations result in movement of said piston at the frequency of such variations to vary the volume of said liquid in said chamber inversely with such variations and at the frequency thereof; another rigid, liquid-filled passage means communicat 1,939,067 Legg Dec. 12, 1933 2,306,372 Banks Dec. 29, 1942 2,644,329 Redfield July 7, 1953 2,859,296 Neu Nov. 4, 1958 OTHER REFERENCES Bridgman: The Physics of High Pressure, G. Bell and Sons, Ltd., London 1949, pages 69, 70, 99 and 100. 

